When memory shortage occurs in a virtual machine realized on a machine, a technique is known to solve the memory shortage by swapping unused memory between virtual machines on the same machine (for example, Japanese Laid-open Patent Publication No. 05-204760). Moreover, a hot migration technique is known to solve the memory shortage, in which a virtual machine in which the memory shortage occurs is moved to another machine having sufficient memory while keeping the virtual machine in operation.
The hot migration technique is superior than the technique disclosed in Japanese Laid-open Patent Publication No. 05-204760 because the hot migration technique can be used not only for solving the memory shortage of the virtual machine but also for solving shortage of other resources such as a computing power of a central processing unit (CPU) and a communication capacity of a network.
However, for moving a virtual machine between machines by using the hot migration technique, a free space enough to store the virtual machine to be moved needs to be prepared in advance in a memory of a destination machine. Therefore, an extremely large memory needs to always be allocated in an unused state, which leads low use efficiency of resources.